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Chapter 2 Atoms, Molecules, and Ions

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1 Chapter 2 Atoms, Molecules, and Ions

2 History Lesson Democritus ( BC): indivisible particles called atoms Plato and Aristotle challenged this view believing that matter was continuous Newton ( AD) proposed the idea of invisible particles in the air called atoms Antoine Lavoisier ( AD) conducted experiments demonstrating mass of products = mass of reactants John Dalton ( AD) proposed a model of matter Dalton’s Atomic Theory

3 Dalton’s Atomic Theory
Postulates Assumption: Matter is discontinuous! 1: Matter is composed of atoms. An atom is the smallest unit of an element that has all the properties of that element. 2: An element is composed entirely of one type of atom.

4 Dalton’s Atomic Theory (cont’d)
3: A compound contains atoms of two or more different elements. The relative number of atoms of each element in a compound is always the same. 4: Atoms do not change identity in chemical reactions; only the way in which they are joined together changes.

5 Law of Constant Composition
Law of constant composition: All samples of a pure substance contain the same elements in the same proportions by mass. This observation follows from Dalton’s third postulate (the relative numbers of atoms are the same in the same compound).

6 Law if Multiple Proportions
Law of multiple proportions: When the same elements form more than one compound, the masses of one element that combines with a fixed mass of a second element are in a ratio of small whole numbers. This follows from the postulate that individual atoms enter into chemical combination.

7 Law of Conservation of Mass
Law of Conservation of Mass: There is no detectable change in mass when a chemical reaction occurs. Dalton’s fourth postulate accounts for this law. The atoms do not change mass or identity when a chemical reaction takes place.

8 Atomic Composition and Structure
Experiments over many years showed that atoms are not simple particles, but are composed of the subatomic particles listed below: Electrons Protons Neutrons

9 Cathode Rays The application of a high voltage across a partially evacuated tube produces cathode rays.

10 Electrons J. J. Thomson demonstrated that cathode rays were negatively charged by applying magnetic and electric fields to cathode rays. Cathode rays are electrons, negatively charged particles that are one of the components of an atom.

11 Millikan Oil Drop Experiment
Robert A. Millikan performed experiments that determined the charge of the electron as 1.60 x coulombs.

12 Mass of electron Thompson, using Milikan’s data for the charge of an electron, determined the mass to charge ratio of an electron. This allowed him to calculate the mass of an electron. 9.11 x kg (mass of electron)

13 Scattering of Alpha Particles by Gold

14 The Nuclear Model of the Atom
Rutherford concluded that the results of the scattering experiment required that atoms consist of: a nucleus that is very small compared to the atom, has a high positive charge and contains most of the mass of the atom. the remainder of the space in an atom contains enough electrons to give a neutral atom.

15 Atomic View of Rutherford Experiment

16 The Proton Rutherford proposed that the hydrogen nucleus was a fundamental particle called the proton, which has a positive charge equal in magnitude to the negative charge of the electron. Protons account for the charge on the nucleus of all atoms. The mass of the proton (1.673 x kg) is 1836 times that of the electron.

17 The Neutron The number of protons in a nucleus, as determined by its positive charge, accounts for half or less of the nuclear mass. Scientists inferred there must be a massive, neutral particle also present in the nucleus. This neutral particle is called the neutron; its mass is almost the same as that of the proton.

18 Particles in the Atom

19 Definitions Atomic number (Z) is the number of protons in the nucleus of an atom. Mass number (A) is the sum of the numbers of protons and neutrons in the nucleus. The number of protons (the atomic number) determines the identity of the element; all H atoms contain 1 proton, all He atoms contain 2 protons, etc.

20 Isotopes Isotopes are atoms of one element whose nuclei contain different numbers of neutrons (same Z, different A). Isotopes of Hydrogen What about Dalton’s Postulate #2?

21 Symbols of Isotopes A symbol to identify a specific isotope is
where A = mass number, Z = atomic number, and X is the one or two letter symbol of the element. The three isotopes of hydrogen are:

22 Symbols of Isotopes Oxygen also has three isotopes, containing 8, 9, and 10 neutrons respectively. The symbols are: Since the value of Z, and the symbol, both identify the element, Z is often omitted from the symbol:

23 Example: Symbols of Atoms
Write the symbol for the isotopes with: (a) 15 protons and 16 neutrons. (b) 21 protons and 24 neutrons.

24 Ions In many chemical reactions, atoms gain or lose electrons, producing charged particles called ions. A cation has a positive charge and forms when an atom loses one or more electrons. An anion has a negative charge and forms when an atom gains one or more electrons. (a)

25 Symbols for Ions The number of protons in the nucleus determines the symbol used for an ion. The element’s symbol is followed by a superscript number and a sign that shows the charge on the ion in electron charge units. If the ionic charge is one unit, the number is omitted, e.g. Na+ is the symbol for a sodium cation.

26 Example: Symbols of Ions
Write the symbol for the ions that contain: (a) 9 protons, 10 neutrons, 10 electrons. (b) 19 protons, 20 neutrons, 18 electrons.

27 Practice Write the symbols for the particles containing: (a) 8 protons, 9 neutrons, 10 electrons (b) 13 protons, 14 neutrons, 13 electrons

28 Test Your Skill Write the symbols for the particles containing: (a) 8 protons, 9 neutrons, 10 electrons (b) 13 protons, 14 neutrons, 13 electrons Answer: (a) (b)

29 Example: Components of Ions
Fill in the blanks. Symbol Atomic number ____ Mass number ____ Charge ____ no. of protons ____ no. of neutrons ____ no. of electrons ____

30 The Atomic Mass Unit (u)
A relative mass scale has been established to express the masses of atoms. The atomic mass unit (u) is 1/12 the mass of one 12C atom. Experimentally to three significant digits: 1 u = 1.66 x kg

31 Masses of Atoms in u The masses of both the proton and the neutron are approximately 1 u. A 24Mg atom has a mass approximately twice that of the 12C atom, so its mass is 24 u. A 4He atom has a mass approximately 1/3 that of the 12C atom, so its mass is 4 u.

32 Atomic Mass and Mass Number
Factors other than the mass of the protons and neutrons affect the mass of atoms, so the actual mass of atoms are not whole numbers. (24Mg = u; 4He = u) When the accurate atomic mass of an atom is rounded to a whole number, it equals the mass number.

33 Natural Distribution of Isotopes
About 75% of the elements occur in nature as mixtures of isotopes. Usually, the relative abundance of isotopes of an element is the same throughout nature. In all natural samples of Li, 7.42% of the atoms are 6Li and the remaining 92.58% are 7Li.

34 Atomic Masses of the Elements
Isotopic mass is the mass in u, of a particular isotope of an element. Different isotopes of an element all react essentially the same, so a weighted average of isotopic masses can be used in calculations. The atomic mass is the weighted average mass, of the naturally occurring element. atomic mass = fractionA x isotopic massA + fractionB x isotopic massB

35 Example: Calculating Atomic Mass
A mass spectrometer was used to determine that gallium is 60.11% 69Ga (isotopic mass = u) and 39.89% 71Ga (isotopic mass = u). Calculate the atomic mass of Ga.

36 The Periodic Table Proposed independently by Dimitri Mendeleev and Lothar Meyer. Periodic table: arranges the elements in rows that place elements with similar properties in the same column. Period: a horizontal row Group: a column - contains chemically similar elements

37 Atomic Number and Atomic Mass
The atomic number and atomic mass for each element is given on the periodic table. 38 Atomic number Sr Atomic mass 87.62

38 Important Groups of Elements
Metal: a material that is shiny and is a good electrical conductor; metallic elements are on the center and left side of the periodic table. Nonmetal: an element that is typically a nonconductor; nonmetals are in the top right part of the periodic table. Metalloid: an element that has properties of both metals and nonmetals.

39 Important Groups of Elements
Representative Elements: the elements in the A groups (1,2, 13-18). Transition Metals: the elements in B groups (3-12). Inner Transition Metals: the two rows of metals (lanthanides and actinides) set at the bottom of the periodic table.

40 Important Groups of Elements
Alkali Metals: soft, reactive metals in group 1A. Alkaline Earth Metals: elements in group 2A. Halogens (salt formers): reactive nonmetals in group 7A. Noble Gases: the stable, largely inert, gases in group 8A.

41 Elements and Biology

42 Molecules A molecule is a combination of atoms joined so strongly that they behave as a single particle. The simplest molecules are diatomic - they contain two atoms.

43 Elements If all the atoms in a molecule are the same, the substance is an element.

44 Molecules If two or more elements form a molecule, it is a molecular compound.

45 Molecular Formulas A molecular formula gives the number of every type of atom in the molecule. The elements present in the molecule are identified by their symbols. A subscript number follows each symbol, giving the number of atoms of that element present in the molecule; the subscript is omitted if only one atom of the element is present. A structural formula shows how the atoms are connected in the molecule.

46 Molecular Formulas

47 Molecular Mass The molecular mass is the sum of the atomic masses of all atoms present in the molecular formula, expressed in atomic mass units (u). The diagram shows the strategy for calculating molecular mass.

48 Example: Calculate Molecular Mass
One substance present in smog is dinitrogen tetroxide (N2O4). Calculate its molecular mass.

49 Practice What is the molecular mass of the fuel propane (C3H8 )?

50 Ionic Compounds An ionic compound is composed of cations and anions joined to form a neutral species. Ionic compounds generally form from the combination of metals with nonmetals. In ionic compounds each cation is surrounded by several anions and vice versa.

51 Structure of Sodium Chloride

52 Formulas of Ionic Compounds
The formula of an ionic compound is an empirical formula that uses the smallest whole number subscripts to express the relative numbers of ions. The relative numbers of ions in the empirical formula balances the charges to zero. The formula of sodium chloride is NaCl, because the 1+ ions have to be present in a 1:1 ratio. The formula of sodium oxide is Na2O, because the charge of the Na+ and O2- ions balance to zero in a 2:1 ratio.

53 Formulas of Ionic Compounds
The position of an element in the periodic table can be used to determine the charges of some ions. The metallic elements in Groups 1A, 2A, 3B, and Al (Group 3A) all form cations with a charge equal to the Group number. The nonmetals in Groups 6A, 7A, and N in group 5A form anions with a charge of 2-, 1- and 3-, respectively.

54 Charges on Common Ions

55 Example: Ionic Compounds Formulas
Write the empirical formulas of the compound formed by (a) the cation of Ca and the anion of Br. (b) the cation of Al and the anion of O.

56 Polyatomic Ions Polyatomic ion: a group of atoms with a net charge that behaves as a single particle. The ammonium ion (NH4+) is the most common polyatomic cation. There are many important polyatomic anions.

57 Some Polyatomic Anions

58 Example: Polyatomic Ions Formulas
Write the formulas of the compounds that contain: (a) the calcium ion and nitrate ion. (b) the ammonium ion and the dichromate ion.

59 Formula Mass of Ionic Compounds
Formula mass is the sum of the atomic masses of all atoms in the empirical formula of an ionic compound. The formula mass of Ca(NO2)2 is: 1(Ca) x = (N) x = (O) x = Formula mass = u

60 Chemical Nomenclature
Chemical nomenclature is the organized system for naming compounds. Some of the basic rules of nomenclature are given here for: Ionic compounds Acids Molecular compounds Organic compounds

61 Naming Ionic Compounds
The name of the cation is given first, followed by the name of the anion. For monatomic ions: the name of the cation is the same as the name of the element. the name of an anion is formed from the name of the element by changing the ending to “ide”. The names given in the table are used to name polyatomic ions.

62 Common Monatomic Anions

63 Naming Cations The elements in groups 1A, 2A, and 3B form only one stable ion, but most other metals form more than one cation. For metals that form more than one cation, a Roman numeral equal to the charge of the ion is shown in parentheses following the name of the element. The name of the compound FeCl3 is iron(III) chloride; that of FeCl2 is iron(II) chloride.

64 Example: Ionic Compounds Names
Name the following ionic compounds: (a) NH4Br (b) Ca(NO3)2 (c) MnSO4 Give the formula of the following ionic compounds: (a) chromium(III) nitrate (b) potassium sulfate (c) ammonium dichromate

65 Acids An acid is a compound that produces hydrogen ions when dissolved in water, and for the present can be considered as hydrogen cations combined with one of the anions already discussed. For example HCl, HNO3 and H2SO4 are all acids in water solution.

66 Naming Acids If the anion name ends in “ide”, change the ending to “ic” and add the prefix “hydro”. This is followed by the word acid.

67 Naming Acids (cont’d) If the polyatomic anion name ends in “ate”, change the ending to “ic”; if it ends in “ite” change the ending to “ous”. This is followed by the word acid.

68 Names of Molecular Compounds
Many molecular compounds have nonsystematic common names; e.g. water (H2O), ammonia (NH3), and methane (CH4).

69 Order of Element Names The order of the elements in the names and formulas of molecular compounds is: The element farther to the left in the periodic table appears first. The element closer to the bottom within any group is first. Hydrogen is first when combined with 6A and 7A elements; it is named second when combined with groups 1A through 5A elements. Oxygen is second, except when combined with fluorine.

70 Numerical Prefixes in Names
Often the same elements form more than one compound. Numerical prefixes are used to give the number of atoms present in the molecule. Number Prefix one mono- (often omitted when first) two di- three tri- four tetra- five penta- six hexa-

71 Example: Naming Compounds
What is the name of the following compounds? (a) H2SO4 (b) SF6 (c) C3O2 (d) TiO2

72 Ionic and Molecular Compounds
Ionic compounds are usually combinations of metals and nonmetals, while molecular compounds usually contain only nonmetals. Ionic compounds are usually hard, brittle solids with high melting points; molecular compounds have lower melting points, and may be liquids or gases at room temperature.

73 Dissociation of Ionic Compounds
Most ionic compounds dissociate into individual cations and anions when dissolved in water. NaCl dissociates into Na+ and Cl- in water.

74 Electrolytes An electrolyte is a substance that produces ions in water solution. Ionic compounds are electrolytes - they conduct electricity when dissolved in water. Ionic compounds heated until they melt to form a liquid also conduct electricity.

75 Nonelectrolytes Water and compounds that dissolve in water as neutral molecules are nonelectrolytes, they do not conduct electrical current. Most molecular compounds are also nonconducting.

76 Electrical Conductivity
Measured conductivity of (a) ionic solids, (b) melted or (c) dissolved ionic compounds and (d) molecular compounds. Melted or dissolved ionic compounds conduct.

77 Three Phases of Bromine
Molecular compounds are frequently gasses or low melting solids.

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